NMR trial models:: experiences with the colicin immunity protein Im7 and the p85α C-terminal SH2-peptide complex

Two cases of successful molecular replacement using NMR trial models are presented. One is the crystal structure of the Escherichia coli colicin immunity protein Im7; the other is a heretofore unreported crystal structure of a specific PDGF receptor-derived peptide complex of the carboxy-terminal SH2 domain from the p85 alpha subunit of human phosphatidylinositol 3-OH kinase. In both cases, molecular replacement was non-trivial. Success was achieved using trial models that consisted of an ensemble of NMR structures from which the more flexible portions had been excized. Use of maximum-likelihood refinement proved critical to be able to refine the poor starting models. The challenges typical of the use of NMR trial models in molecular replacement are discussed.